(Not applicable)
1. Field of the Invention
The invention relates to laser drilling of through-holes in materials.
2. Description of Related Art
Typically, laser-drilling (or laser ablation) of through-holes in various materials results in a hole with the property that the entrance hole (where the laser beam first begins to remove material) has a larger diameter than the exit hole (where the laser beam exits after drilling through the material), thus resulting in a through-hole with tapered sidewalls as shown in FIG. 1. This is especially true for percussion laser drilling in which the laser beam is simply focused on a stationary substrate for the amount of time required to drill completely through the material. The resulting characteristically conical cross-section hole is commonly exploited in various fluid flow applications: the tapered sidewall causes an increase in the velocity of a fluid traveling through the hole in the direction of taper without having to increase the force needed to push the fluid through the hole. To achieve this result, the hole must be fabricated such that the drilling laser travels through the material in the same direction that the fluid will ultimately travel through the material.
For applications in which tapered sidewalls are not desirable, certain optical and/or mechanical methods are commonly employed to approximately equalize the size of the entrance and exit openings of a through-hole, resulting in what is commonly referred to as a high aspect ratio hole, where the aspect ratio is generally defined as the thickness of the material divided by the exit opening diameter. One such common technique for achieving high aspect ratio is known as trepan laser drilling, or laser trepanning. In this method, the material and focused laser spot are moved relative to each other in a closed loop motion (generally circular in the plane of the substrate material) such that a piece of the material is cut out, thereby generating a through-hole of appropriate profile in the substrate. This method can produce through-holes with very steep sidewalls and virtually no taperxe2x80x94that is, with equal entrance opening and exit opening diameters.
Notwithstanding the above, for certain applications and situations a tapered hole is actually desired, but creating one becomes a very difficult task for various reasons, particularly when the direction of taper relative to the impinging laser beam is to be taken into account. One source of difficulty is the presence of obstacles on the side of the material from which the laser must impingexe2x80x94side S1 in FIG. 1. In such a situation, because of the presence of obstacles (not shown) which limit access to side S1, it would be desirable to impinge the substrate from the opposite sidexe2x80x94side S2xe2x80x94such that a counter-tapered through-hole is formed in the substrate. A counter-tapered through-hole, as defined herein, expands away from the drilling laser and therefore has a larger exit opening than entrance opening. Such a hole has heretofore been difficult to drill using conventional laser drilling techniques.
The invention overcomes the shortcomings of the prior art by providing a method of drilling a through-hole in a material having first and second surfaces separated by a predetermined thickness. In accordance with the method of the invention, a laser beam of sufficient energy to ablate the material is directed, along an incidence axis and onto an impingement location on the first surface, with the incidence axis being offset from a normal to the first surface at the impingement location. The material and laser beam are then relatively moved, in a closed path, to produce the desired through-hole profile.
Further in accordance with the invention, a method for drilling counter-tapered through-holes in a fuel injector nozzle having a chamber at least partially defined by a wall having an interior surface adjoining the chamber and an exterior surface opposite to said interior surface is provided. The method in accordance with the invention includes directing, along an incidence axis and onto an impingement location on the exterior surface, a laser beam of sufficient energy to ablate the material of the wall, the incidence axis being offset from a normal to the exterior surface at the impingement location, and relatively rotating the wall and the laser beam such that a through-hole having a larger exit opening than entrance opening is formed.
Further in accordance with the invention, a method of drilling a counter-tapered through-hole in a material having first and second surfaces separated by a predetermined thickness includes directing, along an incidence axis and onto an impingement location on the first surface, a laser beam of sufficient energy to ablate the material, the incidence axis being offset from a normal to the first surface at the impingement location, and relatively moving the material and the laser beam such that a through-hole having a larger exit opening than entrance opening is formed.